Near-field exposure photoresist and fine pattern forming method using the same

ABSTRACT

A near-field photoresist for formation of a fine pattern with by near-field exposure includes an alkali-soluble novalac resin, a diazyde-type photosensitizer which is photoreactive by near-field exposure, a photoacid generator which generates acid by the near-field exposure, and a solvent.

FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an near-field exposure photoresist anda fine pattern forming method using the near-field exposure photoresist.

In recent years, with the progress of an increase in capacity of asemiconductor memory and a high-speed and high-integrated CPU (centralprocessing unit), smaller photolithography has been further required.For this purpose, a wavelength of light used in a photolithographicapparatus is made shorter, and a ultraviolet laser has been presentlyused, thus permitting fine patterning of about 0.1 μm. However, in orderto effect further fine patterning, there are many problems to be solved,such as a shorter wavelength of laser light, development of lens whichcan be used in such a shorter wavelength region, and a compactapparatus.

On the other hand, a method using near-field light has been proposed inorder to effect formation of a photoresist pattern having a width of notmore than a light wavelength by using light.

For example, Japanese Laid-Open Patent Application Hei 7-106229 hasdisclosed a near-field exposure method employing scanning with a probeof optical fiber having a tip which has been sharpened by wet etching.Further, in order to solve a problem of low throughput, U.S. Pat. No.6,171,730 has proposed a simultaneous near-field exposure using a mask.

The near-field exposure has the advantage of determining an availableminimum pattern width depending on a probe used or an opening diameterof a mask used, irrespective of a wavelength of light. Accordingly, itbecomes possible to form a fine pattern of not more than 0.1 μm withoutusing a large-sized and expensive light source, such as an excimerlaser. As an light source for exposure, it is also possible to employ acompact and inexpensive mercury lamp or blue semiconductor laser.

As described above, the near-field exposure is advantageous forrealization of fine pattern without using shorter wavelength light.However, a photoresist is exposed to scattered light which is created bydisturbing near-field light by the photoresist, so that a resultantwidth of a formed pattern is increased when the thickness of thephotoresist for image formation is large. As a result, the pattern widthtends to become larger. FIGS. 4(a) and 4(b) shows such a state. In theseFigures, as a mask, a light blocking film 205 is formed on a mask basematerial 204. Near-field light 510 is created in the vicinity of a smallopening 513 by causing exposure light 505 to enter a mask provided withthe small opening 513 (FIG. 4(a)).

When the mask and a photoresist 503 are close to each other (FIG. 4(b)),the near-field light 510 is scattered by the photoresist 503 on asubstrate 504. As a result, a photoresist reactive portion 501 is formedin the photoresist 503.

In the case where the photoresist 503 has a large thickness, when anamount of exposure is increased in order to effect exposure of thesubstrate in the thickness direction of the photoresist 503, thephotoresist reactive portion 501 is broadened toward the surface of thesubstrate 504, thus providing a larger width of a pattern to be formed.Further, when a spacing between small openings is small, respectivephotoresist reactive portions from associated small openings overlapwith each other, thus further broaden the line width of pattern to beformed. Further, it is necessary to decrease an amount of exposure whenthe fine pattern is intended to be formed. In such a case, however,there is a possibility that the pattern is formed only up to a smallerdepth position of an upper layer portion of the photoresist.

For this reason, in order to form the fine pattern by near-fieldexposure, it is desirable that the photoresist has a thickness smallerthan a minimum opening diameter of an opening through which the exposurelight passes.

However, in the case of the above described thinner photoresist, aresistance to etching is lowered during transfer of pattern onto thesubstrate. Accordingly, when a finer pattern is formed, there is apossibility that a process tolerance is lowered during the patterntransfer onto the substrate or that the pattern transfer onto thesubstrate is failed with respect to such a substrate that it requiresthe use of gas or system providing an insufficient latitude in selectionof the photoresist. SUMMARY OF THE INVENTION

An object of the present invention is to provide a near-field exposurephotoresist capable of effecting transfer of a resist pattern formed ata high contrast and a high aspect ratio onto a workpiece substrate witha process tolerance.

Another object of the present invention is to provide a fine patternforming method using the near-field exposure photoresist.

According to the present invention, there is provided a near-fieldphotoresist for formation of a fine pattern with near-field exposure,comprising: an alkali-soluble novolac resin, a diazyde-typephotosensitizer which is photoreactive by the near-field exposure, aphotoacid generator which generates acid by near-field exposure, and asolvent.

In the photoresist of the present invention, the alkali-soluble novolacresin may preferably be synthesized through condensation of a phenolcomponent with an aldehyde component in the presence of an acidcatalyst. The diazyde-type photosensitizer may preferably be a compoundcapable of being synthesized through esterification between apolyhydroxy compound and quinonediazyde sulfonic acid as aphotosensitive group. The photoacid generator may preferably be an oniumsalt or triazine. The solvent may preferably be a single solvent or amixture solvent selected from the group consisting of propylene glycolmonomethyl ether acetate, ethyl lactate, butyl acetate, and 2-heptanone.

According to the present invention, there is also provided a finepattern forming method for transferring a fine pattern onto a workpiecesubstrate by means of an exposure mask provided with the fine pattern,comprising:

a step of preparing the exposure mask provided with the fine pattern,

a step of preparing a near-field exposure photoresist comprising analkali-soluble novolac resin, a diazyde-type photosensitizer which isphotoreactive by near-field exposure, a photoacid generator whichgenerates acid by the near-field exposure, and a solvent,

a step of disposing the near-field exposure photoresist on the workpiecesubstrate,

a step of silylating an upper layer portion of the near-field exposurephotoresist,

a step of forming a fine pattern at the silylated portion of thephotoresist by effecting the near-field exposure, and a step of formingthe fine pattern on the workpiece substrate on the basis of the finepattern.

According to the present invention, there is further provided a finepattern forming method, comprising:

a step of applying on a workpiece substrate an near-field exposurephotoresist comprising an alkali-soluble novolac resin, a diazyde-typephotosensitizer which is photoreactive by near-field exposure, aphotoacid generator which generates acid by the near-field exposure, anda solvent,

a step of silylating an upper layer portion of the photoresist in avapor phase or a liquid phase,

a step of forming a latent image pattern at the silylated portion of thephotoresist by exposing the silylated portion to near-field light,

-   -   a step of developing the latent image pattern formed at the        silylated portion with alkali developer,

a step of transferring the developed pattern at the silylated portiononto the photoresist in a thickness direction of the photoresist byoxygen dry etching, and

a step of transferring the transferred photoresist pattern onto thesubstrate by dry etching.

In the fine pattern forming method of the present invention, in thesilylating step, the thickness of the silylated portion may preferablybe not more than a size of an opening diameter of a small opening of amask for use in near-field exposure.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. (a) to 1(f) are views for illustrating a fine pattern formingmethod as an embodiment of the present invention.

FIG. 2 is a reaction scheme showing silylation reaction of novolac resinin an embodiment of the present invention.

FIGS. 3(a) and 3(b) are reaction schemes showing a reaction at anexposure portion in an embodiment of the present invention.

FIGS. 4(a) and 4(b) are views showing a state of exposure of a thickerphotoresist to light for illustrating a problem to be solved by thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The photoresist according to the present invention includes analkali-soluble novolac resin, a diazyde-type photosensitizer which isreactive by near-field exposure, a photoacid generator which generatesacid by near-field exposure, and a solvent.

The alkali-soluble novolac resin may, e.g., be prepared by synthesizedthrough condensation between a phenolic component and an aldehydecomponent in the presence of an acid catalyst, dissolving thesynthesized alkali-soluble novalac resin in a polar solvent includingalcohols, such as methanol and ethanol; ketones, such as methyl ethylketone; and cyclic ethers, such as dioxane and tetrahydrofuran, andbeing added in a nonpolar solvent, such as a water-polar solventmixture, heptane or hexane to precipitate a resinous content.Alternatively, it is also possible to prepare the alkali-soluble novalacresin, e.g., by batch-wise or successively adding the aldehyde componentduring the reaction of the phenolic component with the aldehydecomponent to control the condensation reaction.

As the phenolic component used for synthesizing the alkali-solublenovalac resin used in this embodiment, it is possible to use phenol;m-cresol; p-cresol; o-cresol; xylenols, such as 2,5-xylenol and3,5-xylenol; m-ethylphenol; p-ethylphenol; o-ethylphenol;2,3,5-trimethylphenol; butylphenol; hydroquinone;dihydroxydiphenylpropane trimethyl-phenol; propylphenol; anddihydroxybenzene. These phenols may be used singly or in mixture of twoor more species.

As the aldehyde component, it is possible to use formaldehyde,paraformaldehyde, acetaldehyde, benzaldehyde, phenylacetaldehyde, andfurfural. As the acid catalyst, it is possible to use hydrochloric acid,sulfuric acid, formic acid, and oxalic acid.

The above described aldehyde component may be used in an amount of 0.7-3mol per 1 mol of the phenolic component, depending on a reactioncondition. The acid catalyst may generally be used in an amount of1×1⁻⁴-5×10⁻³ mol per 1 mol of the phenolic component. Further, thereaction temperature may ordinarily be 10-200° C., preferably 70-130°C.

The alkali-soluble novalac resin used in the present invention may havevarious substituents so long as they impair an alkali solubility of theresultant resin.

The diazyde-type photosensitizer used in this embodiment is a compoundwhich can be synthesized through an esterification between a polyhydroxycompound and quinonediazyde sulfonic acid as a photosensitive group.

As the polyhydroxy compound, it is possible to use,2,3,4-trihydroxylbenzophenone; 2,3,4′-trihydroxybenzophenone;2,4,6-trihydroxybenzophenone; and 2,3,4,4′-tetrahydroxybenzophenone.

As the quinonediazyde sulfonic acid, it is possible to usebenzoquinone-1,2-diazyde-4-sulfonic acid and naphthoquinone-1,2-diazydesulfonic acids, such as naphthoquinone-1,2-diazyde-4-sulfonic acid andnaphthoquinone-1,2-diazyde-5-sulfonic acid.

As the photoacid generator used in this embodiment, it is possible touse onium salts, such as iodonium salt and sulfonium salt; and triazine.Further, in the case where the photoacid generator alone has a poor acidgenerating efficiency to become problematic, it is possible to use asensitizer, such as 2-ethyl-9,10-dimethyloxyanthracene, in combination.

A photoresist in this embodiment is prepared by dissolving the aboveprepared alkali-soluble novalac resin, the photosensitizer, and thephotoacid generator in a solvent.

As the solvent, it is possible to use any solvent in which thealkali-soluble novalac resin, the photosensitizer and the photoacidgenerator can be dissolved. From the viewpoint of safety, it isdesirable that PGMEA (propylene glycol monomethyl ether acetate), ethyllactate, butyl acetate, and 2-heptanone are used singly or in mixture oftwo or more species.

The above prepared positive photoresist for near-field exposure in thisembodiment may further contain known additives including analkali-soluble resin other than the alkali-soluble novalac resin,another sensitizer, a surfactant, a colorant, an adhesive acid, astorage stabilizer, an antifoaming agent, etc., as desired.

By using the thus prepared photoresist, a fine pattern is transferredonto a substrate in the following manner.

With reference to FIG. 1, a fine pattern forming method in thisembodiment will be described.

On a workpiece substrate 102, a photoresist 101 described above isformed in a layer by using a spin coater or the like (FIG. 1(a)). Thelayer thickness of the photoresist 101 at this time is set so that itcan resist a decrease in thickness during dry etching of the substrate102 with gas with respect to an intended depth of etching of thesubstrate 102.

Then, at the surface of the photoresist 101, silylation is effected(FIG. 1(b)). The silylation may be effected in a vapor phase or a gasphase.

As a silylation agent 103, it is possible to use HMDS(hexamethyldisilazane), DMSDEA (dimethyl-silyldiethylamine), TMDS(tetramethyldisilazane), TMSDMA (trimethylsilyldimethylamine), DMSDMA(dimethylsilyldimethylamine), TMSDEA (trimethylsilyl-diethylamine),HMCTS (hexamethylcyclotrisilaxane), B[DMA]MS(bis(dimethylamino)methylsilane), and B[DMA]DS(bis(dimethylamino)dimethylsilane).

In the case of liquid-phase silylation, a diffusion accelerator and asolvent may be used in combination with the silylation agent describedabove, as desired. As the diffusion agent, it is possible to use DGM(dimethyleneglycol dimethyl ether), NMP (N-methyl-2-pyrrolidone), andPGMEA (propylene glycol methyl ether acetate). As the solvent, it ispossible to use xylene, n-decane and n-heptane.

The depth of pattern which can be formed by near-field exposurecorresponds to approximately a minimum opening diameter of a mask usedat the time of exposure, so that a silylation condition mayappropriately be adjusted to provide the thickness of silylation portion104, i.e., the silylation depth of not more than the minimum openingdiameter. The silylation condition may include the kind of silylationagent, a temperature, an atmospheric pressure, a time, etc., for thevapor-phase silylation, and the kind of silylation agent, aconcentration of silylation agent, a concentration of silylationaccelerator, a temperature, a time, etc., for the liquid-phasesilylation.

By the silylation, in the photoresist 101, it is considered that such areaction shown in FIG. 2 is caused to occur. By this reaction, thesilylation portion 104 is formed at an upper layer portion of thephotoresist 101.

The thus formed silylation portion 104 is subjected to near-fieldexposure (FIG. 1(c)).

In this embodiment, simultaneous exposure using a photomask is employedbut near-field exposure may be effected by using a near-field probe etc.

Referring to FIG. 1(c), incident light 105 incident from an exposurelight source such as a mercury lamp enters a small opening 108 providedm a light blocking film 107 through a mask base material 106. In thevicinity of the small opening 108, near-field light 109 is created. Themask is brought immediately near to the photoresist 101, whereby thesilylation portion gets near to such a range in which the near-fieldlight 109 is present. As a result, it is possible to form a latent imagepattern at the silylation portion 104 as the upper layer of thephotoresist 101.

Here, by the near-field exposure, it is considered that the followingreaction occurs in the silylation portion 104.

By the near-field exposure, acid is generated from the photoacidgenerator, and by the acid, desilylation of the silylated novolac resinis caused to occur (FIG. 3(a)) and the sensitizer is changed into ketene(FIG. 3(b)).

Thereafter, PEB (post exposure bake) is effected. The PEB may beeffected in a water vapor atmosphere as desired.

Then, alkali development is performed. By performing the alkalidevelopment, the following phenomenon occurs.

At an exposed potion, the novolac resin which has been desilylated isdissolved, and the photosensitizer which has been changed into theketene is changed into indene carboxylic acid to be dissolved in analkaline developer. The indene carboxylic acid is considered that itaccelerates the dissolution of he novolac resin, so that the dissolutionspeed in the alkaline developer is increased when compared with the caseof the novolac resin alone.

On the other hand, at an unexposed portion, the silylated novolac resinitself is not readily dissolved in the alkaline developer. Further, anunreacted photosensitizer which has not been exposed to light creates alinking state with the novolac resin to have a dissolution suppressioneffect in the alkaline developer, so that the novolac resin is notfurther dissolved readily in the alkaline developer. Further, when anonium salt is used as the photoacid generator, the onium salt suppressesthe dissolution of the novolac resin in the alkaline developer.

For these reasons, a contrast of image between the exposed portion andthe unexposed portion is increased by the use of the photoresistaccording to the present invention. As a result, in the case where afine pattern of a line width of several ten nanometers is formed bynear-field exposure, it becomes possible to form a fine pattern at arelatively shallow depth-silylation portion 104 with a good contrast.Accordingly, in a subsequent process, it is possible to ensure atolerance.

By the alkali development described above, the latent image pattern isdeveloped by the near-field exposure to provide a surface uneven patternof the photoresist with a good contrast (FIG. 1(d)).

Next, resist etching is effected up to the substrate 102 by dry etchingwith oxygen gas through the surface uneven pattern of the photoresist asa mask (FIG. 1(e)).

The novolac resin at the unexposed portion contains silicon atom by thesilylation, and the silicon atom reacts with oxygen gas to providesilicon oxide, so that an oxygen dry etching resistance is considerablyenhanced compared with the photoresist on the substrate 102 side.Further, the uneven pattern formed at the silylation portion 104 isformed with a good contrast, so that a pattern having a good contrastand a high aspect ratio is also transferred onto the photoresist 110.

By using the above prepared resist pattern as a photomask, etching ofthe substrate 102 with gas capable of etching the substrate 102 iseffected (FIG. 1(f)).

The fine pattern is also well transferred onto the substrate 102 byetching since the resist pattern on the substrate 102 is formed with agood contrast at a high aspect ratio.

EXAMPLE

In order to form a 50 nm L/S (line and space) pattern of a 100 nm-thickupper Si layer on SiO₂ layer of SOI (silicon on insulator) substrate, aphotoresist was formed on the SOI substrate by spin coating.

The photoresist contained an alkali-soluble novalac resin, aphotosensitizer having a naphthoquinonediazyde group, triazine and asensitizer.

With respect to the thickness of the photoresist layer, it is necessaryto provide the photoresist layer on the SOI substrate with a thicknessof to less than 50 nm in view of a ratio of etching resistance betweenSi and the photoresist with respect to a mixture gas of CHF₃ and SF₆since Si is etched with the mixture gas. Further, in order to form the50 nm L/S pattern by near-field exposure, a width of a small opening ofthe photomask used in the near-field exposure is approximately 20 nm.For this reason, a thickness of the silylation portion is determined as20 nm.

When a 20 nm-thick silylation portion is used as the photomask for dryetching, it is possible to perpendicularly etch a 100 nm-thickphotoresist layer with oxygen gas.

Accordingly, in order to ensure a process tolerance during the etchingof Si, the thickness of the photoresist layer applied onto the SOIsubstrate was set to 100 nm in view of the above described photoresistlayer thickness condition.

Thereafter, the entire surface of the photoresist was silylated by aliquid-phase silylation. As the silylation agent, a mixture of HMCTS anda diffusion accelerator dissolved in n-decane was used. The liquid-phasesilylation was effected by immersing the substrate coated with thephotoresist in the liquid-phase silylation agent at room temperature,thus forming a 20 nm-thick silylation portion.

Then, near-field exposure was performed in the following manner.

A photomask having a light blocking portion which was provided with apattern of 100 nm in pitch and 20 nm in small opening width was causedto intimately contact the photoresist at its entire surface, followed byexposure to light by a mercury lamp to form a latent image. Thereafter,PEB was performed in water vapor atmosphere and development wasperformed by immersing the substrate in the alkaline developer.

As described above, the photoresist contained the photoacid generatorand the photosensitizer, so that it was possible to form a patternhaving a good contrast between the exposed and unexposed portions and agood rectangular shape.

By using the photoresist pattern formed at the silylation portion was amask, the photoresist was dry-etched with oxygen gas up to an interfacewith the Si substrate. As a result, since the pattern at the silylationportion was formed with a very good contrast, a good photoresist patternwas formed although the silylation portion had a smaller thickness of 20nm.

By using the resultant photoresist pattern as a mask for dry etching,dry etching of the Si substrate with a mixture gas of CHF₃ and SF₆ wasperformed. Since the photoresist pattern was very good, it was possibleto form a 50 nm L/S pattern having a depth of 100 nm on the insulatingmaterial with a tolerance for the dry etching process of Si.

While the invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth and thisapplication is intended to cover such modifications or changes as maycome within the purposes of the improvements or the scope of thefollowing claims.

This application claims priority from Japanese Patent Application No.298973/2003 filed Aug. 22, 2003, which is hereby incorporated byreference.

1. A near-field photoresist for formation of a fine pattern withnear-field exposure, comprising: an alkali-soluble novalac resin; adiazyde-type photosensitizer which is photoreactive by the near-fieldexposure: a photoacid generator which generates acid by the near-fieldexposure; and a solvent.
 2. A photoresist according to claim 1, whereinsaid alkali-soluble novalac resin has been synthesized throughcondensation of a phenol component with an aldehyde component in thepresence of an acid catalyst.
 3. A photoresist according to claim 1,wherein said diazyde-type photosensitizer is a compound capable of beingsynthesized through esterification between a polyhydroxy compound andquinonediazyde sulfonic acid as a photosensitive group.
 4. A photoresistaccording to claim 1, wherein said photoacid generator is an onium saltor triazine.
 5. A photoresist according to claim 1, wherein said solventis a single solvent selected from the group consisting of propyleneglycol monomethyl ether acetate, ethyl lactate, butyl acetate, and2-heptanone, or is a mixture solvent comprising two or more solventsselected from the group consisting of propylene glycol monomethyl etheracetate, ethyl lactate, butyl acetate, and 2-heptanone. 6.-8. (canceled)